When Facebook killed off a standalone location feature called Places in 2011, a lot of people concluded it was giving up the fight against Foursquare's check-in app.

But Facebook pulled a judo move against Foursquare, weaving location into status updates, photos, and other core parts of the social network.

It also very quietly took on Yelp, by letting users recommend places to friends.

It's woven all of that together into Nearby, which pulls together all those moments tagged with location data into a real-time recommendation service.

Josh Williams, the cofounder of Gowalla, a one-time Foursquare rival Facebook acquired in 2011, has been leading the team working on the project.

The service has some bugs. Where Yelp, for example, knows that nearby restaurants have tacos on the menu based on user reviews and other data, and can serve them up when you search for "tacos," Facebook's Nearby doesn't pick up on that kind of information, according to Constine.

Instead, it does a very basic search on names and categories.

There's obviously room for improvement, but given Facebook's huge base of mobile users, this should be a reason for Foursquare and Yelp to worry. And indeed, Yelp's stock fell 3% today.